To find the value of f(g(8)), we will first need to compute g(8) and then use that result to compute f(g(8)).

1. **Calculate g(8)**:
The function g(x) is defined as g(x) = x + 5. Now we substitute x = 8:
g(8) = 8 + 5 = 13.

2. **Calculate f(g(8))**:
We found that g(8) = 13. Now we will calculate f(13) using the function f(x) = 3x:
f(13) = 3 * 13 = 39.

Finally, the value of f(g(8)) is 39.